[media] cx231xx: don't use dev it not allocated

changeset 5eeb3014 added a fixup at the error check
code. However, it introduced a new error:

	drivers/media/usb/cx231xx/cx231xx-cards.c:1586 cx231xx_usb_probe() error: we previously assumed 'dev' could be null (see line 1430)

This happens when dev = kmalloc() fails. So, instead of relying
on it to succeed, just change the parameter of clear_bit() from
'dev->devno' to 'nr'.

Cc: Alexey Khoroshilov <khoroshilov@ispras.ru>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>